Whenever I try to customise my theme, I keep getting this error message where the live site should be. Any help would be appreciated.

Warning: DOMDocument::loadXML() [domdocument.loadxml]: Empty string supplied as input in /websites/123reg/LinuxPackage21/mc/oo/kp/mcookphotography.co.uk/public_html/blog/wp-content/themes/ePix/lib/inc/classes/slideset-class.php on line 154

Fatal error: Call to a member function getElementsByTagName() on a non-object in /websites/123reg/LinuxPackage21/mc/oo/kp/mcookphotography.co.uk/public_html/blog/wp-content/themes/ePix/lib/inc/classes/slideset-class.php on line 157